Emergency vehicles such as police, ambulance and fire engines are usually fitted with warning means in the form of lights and sirens for alerting other road users to their proximity, both for the safety of the other road users and also so that the emergency vehicle may be given right of way.
A problem with lights and sirens is that they are often ineffective in alerting other road users of the proximity of emergency vehicles for a number of reasons. For example, sirens may not be heard due to high ambient noise levels. Furthermore, modern cars are often equipped with stereo systems and air-conditioning in which case the driver may be insulated, and hence oblivious, to external events. Similarly, lights can be ineffective in daylight, smoke, heavy rain or fog, and furthermore lights are reliant on line of sight and attentiveness on the part of the driver, particularly in the monitoring of driving mirrors.
It has been previously proposed that a transmitter/receiver arrangement be utilised to overcome the above noted ineffectiveness of conventional warning means, ie lights and sirens. Specifically, the emergency vehicle can carry a transmitter which will send a signal which is received by a receiver disposed in the vehicle of a normal road user, the receiver being adapted to generate a signal readily perceivable to the operator of the vehicle.
However for both practical and regulatory reasons it is necessary that transmitters associated with different emergency vehicles transmit at the same frequency. Moreover, for reasons of system integrity and to avoid false alarms, it is necessary that the signal transmitted by the transmitter be coded in some way to ensure that the receiver only warns a driver in response to transmissions originating from emergency vehicles.
Accordingly, a situation is likely to arise, for example when fire, police and fire engines all attend a road accident, where a plurality of transmitters are likely to be operative in a common area. Similarly, there may exist situations where signals, not being signals from other emergency vehicles, are present on the relevant radio frequency. This creates a potential problem wherein the coded signal is superimposed and hence corrupted to a point where the receiver does not recognize the coded signal and accordingly no warning is given to the operator of the vehicle.
It has been proposed that the risk of superimposition of coded signals originating from different emergency vehicles be minimized by transmitting the coded signals in pulses, i.e., the coded signal is transmitted periodically with the majority of the period being "silent". This allows a psuedo-interleaving or random time division to take place and reduces, but does not eliminate, the likelihood of superimposition and hence corruption of coded signals.
Such a system is disclosed in the present applicant's Australian Patent Application 76391/87 which is incorporated herein by cross-reference.
Whilst the proposal of 76391/87 offers a partial solution, there remains a real likelihood of superimposition and corruption of the coded signal due to the presence of other non-emergency-vehicle-originating signals or other coded signals from emergency vehicles.